The size requirement of the primary crusher is a function of grizzly openings, ore chute configuration, required throughput, ore moisture, and other factors. Usually, primary crushers are sized by the ability to accept the largest expected ore fragment. Jaw crushers are usually preferred as primary crushers in small installations due to inherent mechanical simplicity and ease of operation of these machines. Additionally, jaw crushers wearing parts are relatively uncomplicated castings and tend to cost less per unit weight of metal than more complicated gyratory crusher castings. The primary crusher must be designed so that adequate surge capacity is present beneath the crusher. An ore stockpile after primary crushing is desirable but is not always possible to include in a compact design.
Many times the single heaviest equipment item in the entire plant is the primary crusher main frame. The ability to transport the crusher main frame sometimes limits crusher size, particularly in remote locations having limited accessibility.
In a smaller installation, the crushing plant should be designed with the minimum number of required equipment items. Usually a crushing plant which can process 300 to 500 metric tons per operating day will consist of a single primary crusher, a single screen, a single secondary cone crusher, and associated conveyor belts. The discharge from both primary and secondary crushers is directed to the screen. Screen oversize serves as feed to the secondary crusher while screen undersize is the finished product (see Figure 1). For throughputs of 500 to 1,000 metric tons per operating day (usually 2 shifts), a closed circuit tertiary cone crusher is usually added to the crushing circuit outlined above. This approach, with the addition of a duplicate screen associated with the tertiary cone crusher, has proven to be effective even on ores having relatively high moisture contents. Provided screen decks are correctly selected, the moist fine material in the incoming ore tends to be removed in the screening stages and therefore does not enter into subsequent crushing units (see Figure 2).
All crusher cavities and major ore transfer points should be equipped with a jib-type crane or hydraulic rock tongs to facilitate the removal of chokes. In addition, secondary crushers must be protected from tramp iron by suspended magnets or magnetic head pulleys. The location of these magnets should be such that recycling of magnetic material back into the system is not possible.
Crushing plants for the tonnages indicated may be considered to be standardized. It is not prudent to spend money researching crusher abrasion indices or determining operating kilowatt consumptions for the required particle size reduction in a proposed small crushing plant. Crushing installations usually are operated to produce the required mill tonnage at a specified size distribution under conditions of varying ore hardness by the variation of the number of operating hours per day. It is normal practice to generously size a small crushing plant so that the daily design crushing tonnage can be produced in one, or at most two, operating shifts per working day.
The screen or screens in a small crushing plant should be oversized and of the four-bearing type if possible. Installed screens should be standard sized and, if multiple screens are used, all screen panels should have the same dimensions. The screen layout should be such that screen decks are readily accessable. Screens should be located for optimum visibility and required chutes should be designed with the maximum open area possible. To minimize the height of the undersize chute, discharge conveyors should be oriented along the long dimension of the screen. The slope of the discharge conveyor should be as nearly parallel to the slope of the screen as is possible. The utilization of rubber top decks should be considered due to the longevity of this material. The use of tall chutes should be avoided or, if absolutely necessary, should be provided with dead beds and cleanout ports. Spare screen panels should be provided with the original equipment since considerable time can lapse between ordering panels and actual receipt, particularly for rubber covered decks.
Although design calculations may indicate overcapacity, no conveyor in any crushing plant should be less than 0.5 meters wide and, in actuality, it is best to standardize on a minimum belt width of 0.67 meters throughout the crushing system. Design should be such that conveyor troughing and return idlers can be lubricated from one side. Barely are small conveyors designed and manufactured with walk-ways on both sides of the belt. In smaller installations some designers prefer to specify permanently self-lubricated idlers. The design of tail pulleys should be oriented toward ease of cleanup. The lowest point of any belt should be at least 0.6 meters above the floor. Rock deflectors or plows located immediately in front of tail pulleys above the returning belt can serve to prevent rooks from cycling between the belt and pulley thereby increasing belt life. In tunnels, beneath stockpiles, and in other locations below grade, provisions must be made to include the cleanup sumps and pumps. In addition, these locations should be protected from flooding. Counterweights and/or mechanical takeups must be positioned in accessable locations for ease of adjustments. Usually gravity takeups are preferred since they tend to be self-adjusting and minimize sideways belt movement. Conveyor layout must take into account wind direction and, for elevated conveyors, covers should be considered. Naturally, all conveyors must be equipped with emergency shutdown switches and all drives, sprockets, and gear trains must be provided with adequate guards. The emergency stop cable should run the length of the belt on the operating side. Conveyor interlock systems must be carefully designed and must be understood by all operational personnel.
In the recent past, conveyor equipment suppliers have indicated an interest in quoting completely engineered and prefabricated conveyor systems, rather than components only. The design engineer must carefully consider these packaged systems since overall costs can sometimes be considerably reduced using these pre-fabricated conveyors rather than custom designed assemblies.
There is a tendency in small plants to overlook or undersize the dust collection system. This can prove to be a serious problem and will usually result in the handling of very wet ore in the fine ore bin since the usual “quick fix” for a dusting problem is the installation of numerous crudely fabricated water sprays. It is best to consider spray locations at all conveyor transfer points in the initial design. It is usually difficult to precisely estimate the dusting problem which will be encountered and, consequently, a selection of interchangeable spray nozzles having different orifices should be supplied as original equipment.
Fine Ore Bins
Fine ore bins must be sized to provide the surge capacity required for continuous operation of the flotation concentrator. The usual configuration for plants processing 500 metric tons per day or less is a single cylindrical bin, 10 meters in diameter and 10 meters high, fabricated of rolled plate which is welded or bolted on site. This type bin is discharged through a slot which is approximately as long as the diameter of the bin. Under no circumstances should the discharge slot in the bin be less than 0.6 meters wide. A 1 meter wide slot the length of the bin discharged by a 1.25 meter wide variable speed conveyor has proven to be an excellent system. Even though this installation appears to be overdesigned, it will pay for itself the first time that the ore becomes wet and/or sticky. If the crushing plant and fine ore bin are in close proximity and if the typography is favorable, one conveyor can be eliminated if the finishing screen is located atop the bin (see Figure 3). In this design, adequate access must be provided for screen maintenance. For larger plant throughputs, from 500 up to 1,000 metric tons per day, a similar sized bin is adequate for a single line plant if the crushing installation is operated 6 days per week. In the case of a 1,000 metric tons per day plant having two 500 metric ton per day grinding lines, a duplicate fine ore bin should be supplied.